With the development of communication systems, various modulation modes with high spectral efficiency have found wide applications. Modulation signals of these modulation modes have inconstant envelopes, thus putting very high demand on the linearity of the power amplifier inside the transmitter. As a result, the linearization method of power amplification becomes an indispensable technique. Due to its simplicity, flexibility and low cost for implementation, digital baseband predistortion has become an efficient linearization method for power amplification.
The predistortion method using scalar information (such as the out-band power and the ratio between the in-band and out-band powers of a power amplified output signal) as the optimizing target can avoid the influence of factors such as delay of the feedback loop and I/O imbalance etc., whereby the process is further simplified and made more effective.
FIG. 1 is a block diagram illustrating the principle of a conventional predistortion method. As shown in FIG. 1, after baseband information source information from an information source 101 passes through a lookup table predistorter 102, an amplitude predistortion value and a phase predistortion value are obtained; a multiplier 103 multiplies the baseband information source information with the predistortion values to obtain a predistorted signal, and this signal is converted by a digital-to-analog converter 104 into an analog signal. After the analog signal is up-converted by an up-converter 105 into an RF signal, it is input to a power amplifier (also referred to as PA) 106. The signal amplified by the power amplifier 106 is transmitted via an antenna. Meanwhile, a part of the signal output from power amplification is down-converted by a down-converter 107 and then fed back to an analog-to-digital converter 108, by which the signal is sampled to obtain a feedback digital baseband signal output from power amplification. The baseband signal is analyzed in an out-band power calculating module 109 using the digital signal processing technology to thereby obtain an out-band power value of the digital baseband signal. Subsequently in a module 110, the out-band power value serves as a target function to optimize parameters of the lookup table predistorter 102, and the parameters of the predistorter are updated using an optimization algorithm to realize the adaptive predistortion process.
One of the essential problems in such algorithms is which kind of structure and algorithm should be used to realize the lookup table predistorter as well as the update of the predistortion parameters used therein.
In certain conventional technologies, a set of predistortion parameters is calculated by measuring power amplification characteristics under various environment conditions, is subsequently stored in a predistorter, and the corresponding predistortion parameter is selected in accordance with the current operating status. However, these methods are inferior in flexibility, and require larger storage capacity and testing operations. The U.S. Pat. No. 6,731,168 prestores two lookup tables, one under high temperature condition and the other under low temperature condition. Interpolation factors are adaptively controlled in accordance with temperature information as fed back, and interpolation is performed between the two lookup tables to obtain the current predistortion table. This method enables the parameters of the predistorter to lie within the range of [0, 1], and this greatly facilitates hardware implementation. Moreover, the predistortion table in the adaptive updating process is always restricted between the high temperature lookup table and the low temperature lookup table, where there is no divergence phenomenon, and the performance is stable. However, this method requires a power amplification heat model to generate interpolation factors, and such a heat model is difficult to obtain in practice. Moreover, under the same power amplification temperature, interpolation factors at each of the lookup table index values are identical, and the predistortion table obtained through interpolation is relatively less in degree of freedom and inferior in flexibility.